This invention relates to a beam position detecting device for optical disc apparatus.
An optical disc apparatus causes, for example, a laser beam from a laser beam source to illuminate a rotating optical disc, while moving in a radial direction of the optical disc, whereby the recording and reading of information on and out of the optical disc are performed. For example, an optical video disc player is known as such optical disc apparatus. With the optical video disc player a so-called random access system is adopted in which track information, i.e. desired frame information, recorded on the disc is reproduced by the laser beam moving in the radial direction. In order to perform a random access operation it is necessary to detect a laser beam spot position on a disc. In order to detect the beam spot position a light beam device provided with a laser, objective lens, reflection mirror etc. includes an electrical or a mechanical displacement measuring device such as a slide type resistor, magnetic sensor (magnetic scale) etc. In the optical video disc player having a random access function in particular, a laser beam is frequently moved for track searching, causing greater wear of the mechanical section of the displacement measuring device and incurring a durability problem for the disc player. In order to obtain a short access time it is necessary to make the movable section much lighter. Moreover, it is also necessary to make the load of the sensor section for position detection as small as possible. With the above-mentioned detection device it would be difficult to satisfy such requirements. It is known to use a linear encoder for detecting moire fringes. Such an arrangement is, however, complicated and expensive and requires a complicated adjustment.